A buck then boost or boost then buck converter, both possible with the same controller, is a type of DC/DC converter whose output voltage may be either higher than or lower than the input voltage in order to meet the requirements of a particular application. The converter selectively operates in a step-down buck mode and in a step-up boost mode. Buck then boost or boost then buck converters regulate voltage in a wide variety of applications including consumer electronics, power amplifiers, self-regulating power supplies, and control applications. The design of a four quadrant buck-boost, or two quadrant, buck then boost or boost then buck converter is similar to a buck converter and boost converter, except that it is in a single circuit and it usually has an added control unit. The pulse width modulator (PWM) control units sense the level of output voltage and current passing through the control switch and regulates output voltage. Input voltage may also be used in the control loop. Input voltage is also used for enabling and disabling the PWM control loop. The control unit takes appropriate action to define circuit paths to produce the buck-boost effect. A buck-boost, buck then boost or boost then buck circuit allows bidirectional current and power and converts voltage in either direction, based on sensed inputs. A buck-boost converter is a four quadrant converter and can also be just a two quadrant converter, however in a buck-boost two quadrant implemented converter two switches of the four switches are not needed for pulse width modulation (PWM) operation in opposed quadrant two quadrant operation. Thus this two quadrant, buck then boost or boost then buck (either allowed, but not in one implementation) topology is more economical to implement than a four quadrant converter. A two quadrant converter only allows boosting in one direction and bucking in the other direction while allowing two directions of current flow. A buck-boost converter in either the input to output direction or the output to input direction can also buck or boost in either direction and allow current to flow in two or opposing directions.
The switching circuits for connection or commutation of current are directly coupled to input and output circuitry. An upstream or downstream fault, or a voltage transient coming from outside the protection switches of the IC can damage the IC. There is not adequate protection of the circuit within the buck-boost converter itself for over voltage protection of the IC and especially of the unprotected PWM switches. The protection means and PWM switches when external to the IC allow for higher currents and voltage withstand via component selection. When implemented inside the IC this is fixed at a static current and voltage level, based on the voltage and temperature limits of the IC.
In the prior art, where a single integrated circuit is used to embody a switch mode one, two, three or four quadrant converter, the switch mode regulator will be incomplete. In order to provide protection and other functions, external components are typically required. Where any current is consumed, the series switch connecting an input or output to a reservoir capacitor or energy storage means, not the PWM/pulse frequency modulation (PFM) dedicated switches, will comprise an external switch. External switches far from either the protection components or control units will create transients due to inductance designed in or must be dealt with such as cabling inductance increase the transient voltage levels beyond what normal system voltages and either separate protection means needs will increase (passive, active, or both). These components require space on a circuit board and additional expense in component cost, space, weight, and assembly. Transient voltages that may be caused by a distant external switch must be prevented from affecting other components in the circuit board. Transient protection shunt devices are also used when needed in conjunction with filters or active protection means to protect the system. These transient voltage devices have limitations in cycle life and voltage and current maximum limits. This may require larger filters for the shunt devices to have an acceptable lifetime of service. This still does not protect the TVS and downstream system in an extreme transient event, even if infrequent, because only one extreme transient is needed to cause failure. Filters take up space and have limitations and stability degradation of the converter, either limiting the filtering needed or forcing slowing down of control loops or both and thus degrade system performance especially stability and voltage compliance or sacrificing survivability limits. Protection means controlled by the combined protection and PWM/PFM controller and their close proximity to the IC allow elimination or reducing filters and shunt protection means.
Many systems using integrated circuits comprising pulse width modulator/pulse frequency modulator (PWM/PFM) converters require protection at the input or output of the integrated circuit (IC) from overvoltages from transients or improper applied voltage sources, including opposing polarity source relative to allowed voltage ranges of system and IC. Prior art voltage conversion ICs have not been designed to include the level of integration of voltage conversion and protection disclosed herein. Consequently, higher levels of protection had to be provided from separate ICs, or discrete (passive, active, or both included) component designs outside of the voltage conversion means. In space and airborne applications, even the weight of an IC and the supporting PWB area and mechanical support is significant. In addition there is a weight multiplier of the total system both in added structure weight and also fuel needed to provide propulsion for this added weight. The additional fuel that is required adds additional structure weight that increases the weight multiplier. Additional ICs or discrete circuits take additional area on a circuit board and require peripheral components. This space and the weight added reduces the efficiency via the added weight. This adds both capital cost to the power converter and the system it supports, but also operating cost due to lower efficiencies of added weight that requires higher fuel burn. Although the added weight is not a big a problem, the added capital costs in ground transportation applications is a problem, just as it is in space and air transportation. Also there is an increase in terms of cost in fixed products such as grid tied energy storage systems.
U.S. Pat. No. 8,415,933 discloses a buck or boost DC/DC converter which provides pulse width modulation (PWM) to determine the buck duty cycle or the boost duty cycle. In a switching circuit, first and second transistors control charging or discharging during a boost operation or buck operation. Another pair of transistors controls direction of power flow. The switching circuit is not configured to provide overvoltage protection. Additional circuitry in another IC must be used.
U.S. Pat. No. 4,736,151 discloses a bidirectional DC/DC converter. First and second power switches are provided for buck and boost operation. Charging capacitors and the transistor power switches are controlled by an integrated circuit. Because the integrated circuit is external to the switching circuits and charging capacitors, the converter is subject to the shortcomings of extra board space being required for discrete components.
U.S. Pat. No. 6,788,033 discloses a buck-boost converter in an integrated circuit. The integrated circuit does not include the required inductor. The discrete inductor is subject to noise and requires additional space on a printed circuit board.
U.S. Pat. No. 8,159,200 discloses an integrated circuit for use in a DC/DC converter. The circuit has to change connection of the inductor in order to account for different operating modes. A pulse width modulation circuit is not provided for commutation of switches.